The present invention relates to an image forming apparatus for copying an image on a document onto a sheet.
In an electronic copying machine of this type, a document is exposed and light reflected thereby is radiated onto a photosensitive drum, to form an electrostatic latent image thereon. Toner is attached to the latent image on the photosensitive drum, to form a toner image which is then transferred onto a sheet. Then, the sheet is peeled from the photosensitive drum, and toner is fixed to the sheet.
When a toner image formed on the photosensitive drum is transferred to a sheet, the sheet is attracted to the peripheral surface of the drum by an electrostatic force. Therefore, when the sheet is peeled from the peripheral surface of the drum, the electrostatic force on the peripheral surface of the drum is removed by a deelectrifying lamp. Then, the sheet is separated from the peripheral surface of the drum due to the inherent stiffness of the sheet (i.e., the force by which a bent sheet returns to the straight state). Toner is heated and melted to be fixed to the sheet.
In recent years, a multiple-copying technique is known, for forming a color image by repeating the above-mentioned copying operation for a single sheet.
In addition, a double-side copying technique is known, wherein after an image on a document is formed on one surface of a sheet, an image on another document is formed on the other surface of the sheet.
In such a multiple-copying or double-side copying technique, after a sheet is fed to a photosensitive drum and a toner image is formed on the sheet, the sheet is again fed to the photosensitive drum, to form a plurality of toner images thereon.
However, once a copying image is formed on a sheet, the stiffness of it is decreased since it is heated during the toner-fixing process, and, as a result, the sheet is often curled or bent.
When the curled sheet is again fed to the photosensitive drum, to form another image thereon, due to the toner-fixing process and the sheet's consequent decreased stiffness, the sheet must now be peeled off the peripheral surface of the drum. However, because the electrostatic force is removed only by the deelectrifying lamp, due to its reduced stiffness, the sheet cannot be reliably peeled off the peripheral surface of the drum. Because of this, paper jamming is likely to occur.
For this reason, another copying machine has been developed, in which a tape is interposed, in advance, between the sheet and the drum, and the sheet is mechanically peeled by off means of the tape.
However, when the tape is interposed between the drum and the sheet, toner cannot be transferred to that portion of the sheet facing the tape. Therefore, when a tape is used, the image-forming area of a sheet is decreased.